Mixing and application capsules are generally used for mixing two or a plurality of components, preferably a solid component and a fluid component, for example in order to produce a dental preparation, which is applied from the mixing and application capsule into a region within the oral cavity of a patient. In this way, for example glass ionomer cement is produced by mixing a powder component with a liquid component.
From the prior art, various devices and techniques are known that are devoted to accommodation of the solid component and of the fluid component within the mixing and application capsule, in order to achieve equally satisfactory mixing behavior and application behavior.
The liquid is for example accommodated in a film bag, which is opened by bursting. The film bag is then expressed, to force the liquid out of it and supply it to the pulverant component. This method has the drawback that during expressing, in particular with a bag that is very long relative to its diameter, the film forms folds, in which an indefinite residue of liquid remains. The result is that the dental preparation paste is mixed with insufficient liquid, so that it does not have the desired properties. Therefore either the size of the film bag and therefore the amount of liquid are restricted or else the dosage is inaccurate. The diameter of the film bag cannot as a rule be varied at will, as the capsule would need a correspondingly larger diameter. A capsule of large diameter is, however, disadvantageous during application in a patient's oral cavity. Moreover, the tongs used for expressing the mixing and application capsule are usually designed for small capsule diameters.
In one variant, a film bag is arranged laterally on or at the capsule body. Such a bag can therefore have a much larger diameter and amount of liquid. However, there is the disadvantage that a separate activating tool is required, for opening the film bag and bringing its contents in the capsule in contact with the powder (i.e. to activate the capsule). Also, proportionally more liquid is left behind in a bag with large diameter.
Another example of accommodation of a liquid in a mixing and application capsule for producing a dental preparation is described in U.S. Pat. No. 6,386,872 B1. A first and a second piston are arranged inside the capsule body, and a liquid is arranged in the space between them. The first piston delimits the mixing space inside the piston body, in which the pulverant component is arranged. The piston wall between mixing chamber and liquid space has a predetermined break point. If the second piston is pushed towards the mixing chamber, a projection on the second piston breaks through the predetermined break point of the wall of the first piston, so that the liquid flows into the mixing chamber, where it mixes with the pulverant component. After activation, for example by pressing the capsule by hand on a table top, the capsule must be clamped in a mixer, which mixes the powder and liquid by shaking, to form a paste.
Then the capsule is clamped in expressing tongs, with the aid of which both pistons are moved forward further. The paste is expelled through the cannula and for example fills a cavity of a tooth being treated.
The arrangement according to U.S. Pat. No. 6,386,872 B1 has the disadvantage that diffusion losses of the liquid occur and therefore the mixing and application capsule is not suitable for storage for a prolonged period. The seal between the first and second piston is a rubbing seal. If the seal is selected to be easy-running, so that a manual displacement of the second piston relative to the first piston is possible, there is inadequate sealing and the liquid can evaporate between the first and second piston. If the seal is selected to be so tight that evaporation of the liquid through the seal is avoided almost completely, the second piston can only be displaced relative to the first piston by applying a large force, which requires special tools and is no longer possible manually. Moreover, there is a similar problem of evaporation of the liquid for the seal between the first piston and the capsule body. The pulverant component in the mixing space is hygroscopic and therefore tends to take up moisture that penetrates through the seal between the first piston and the capsule body. This also impairs the long-term storage capability of the mixing and application capsule. In addition, the thin-walled region with the predetermined break point of the wall of the first piston can easily be penetrated by the liquid. The pulverant component can thus take up moisture from the liquid not only via the seal between first piston and capsule body, but also through the region with the predetermined break point of the wall of the first piston.
A mixing and application capsule that tackles and solves the problems of the rest of the prior art is known from DE 10 2009 016 862. The device known from DE 10 2009 016 862 is based on the double piston principle, wherein a first piston body is a double piston, and between the first piston body and the second piston body, a cavity is formed whose contents are forced out of the cavity through a channel when pressure is applied on the first piston body. If for example a fluid component is stored in the cavity and there is another mixture component, for example a powder component, in a mixing space connected to the cavity by the channel, motion of the double piston leads to mixing of the two components in the mixing space.
The manner of operation of the device known from DE 10 2009 016 862 is already satisfactory. However, in our own investigations of mixing and application capsules of the type known from DE 10 2009 016 862, in extreme test conditions sometimes the problem arose that small amounts of the mixture component from the mixing space get into the channel and pass through it towards the cavity. This has the effect that, in the extreme test conditions, occasionally with application of pressure and the concomitant forcing of the fluid component out of the cavity, mixing of the fluid component with the mixture component occurs in the mixing space even before the fluid component passes through the channel. This can theoretically lead to clogging of the channel, or—if the mixture component has set in other zones—to blocking of the movement of the first and/or second piston body, so that complete mixing of the two components and at worst also the complete expulsion of the dental preparation are made more difficult or are prevented.
Another problem that might sometimes arise with the known mixing and application capsule is that sometimes at the end of the phase of application of pressure and therefore at the end of expulsion of the dental preparation from the mixing space, liquid still flows out of the mixing and application capsule. This effect arises because to expel the paste, a relatively large force is applied, which is much higher than that required for activating the capsule, and thus for expelling the liquid. If activation is not carried out sufficiently completely, a small residue of liquid remains, which is then only discharged as a result of the large force during the application of pressure, i.e. during the operation of application.